Articular disc prosthesis and method for implanting the same

ABSTRACT

An articular disc prosthesis and method of implanting the same within an intervertebral space between adjacent vertebral bodies. The prosthesis includes a pair of articular components and an articular ball disposed therebetween. Each of the articular components includes an outer shell portion and a removable inner insert portion. The insert portion includes a concave articular surface sized and shaped to receive a portion of the articular ball to provide articulating motion between the articular components. The outer shell portion includes a central hemi-cylindrical portion, a pair of laterally extending flanges, and an axially extending lip. Following removal of the natural intervertebral disc, a pair of hemi-cylindrical recesses are formed along a central region of the adjacent vertebral bodies to a predetermined depth. The prosthesis is implanted within the prepared disc space by axially displacing the hemi-cylindrical central portions of the articular components along the hemi-cylindrical recesses in the vertebral bodies. The lateral flanges and the axial lip of the articular components bear against the endplates of the adjacent vertebral bodies to stabilize the prosthesis and to prevent subsidence.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of Provisional ApplicationSer. No. 60/375,354 filed on Apr. 25, 2002, the contents of which arehereby incorporated by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates generally to the field of spinal implants,and more particularly relates to an articular disc prosthesis and methodof implantation for use in the total or partial replacement of a naturalintervertebral disc.

BACKGROUND OF THE INVENTION

In the treatment of diseases, injuries or malformations affecting spinalmotion segments, and especially those affecting disc tissue, it has longbeen known to remove some or all of a degenerated, ruptured or otherwisefailing disc. In cases involving intervertebral disc tissue that hasbeen removed or is otherwise absent from a spinal motion segment,corrective measures are indicated to insure the proper spacing of thevertebrae formerly separated by the removed disc tissue.

In some instances, the two adjacent vertebrae are fused together usingtransplanted bone tissue, an artificial fusion component, or othercompositions or devices. Spinal fusion procedures, however, have raisedconcerns in the medical community that the bio-mechanical rigidity ofintervertebral fusion may predispose neighboring spinal motion segmentsto rapid deterioration. More specifically, unlike a naturalintervertebral disc, spinal fusion prevents the fused vertebrae frompivoting and rotating with respect to one another. Such lack of mobilitytends to increase stresses on adjacent spinal motion segments.Additionally, several conditions may develop within adjacent spinalmotion segments, including disc degeneration, disc herniation,instability, spinal stenosis, spondylosis and facet joint arthritis.Consequently, many patients may require additional disc removal and/oranother type of surgical procedure as a result of spinal fusion.Alternatives to spinal fusion are therefore desirable.

Several different types of intervertebral disc arthroplasty devices havebeen proposed for preventing the collapse of the intervertebral spacebetween adjacent vertebrae while maintaining a certain degree ofstability and range of pivotal and rotational motion therebetween. Suchdevices typically include two or more articular components that areattached to respective upper and lower vertebrae. The articularcomponents are anchored to the upper and lower vertebrae by a number ofmethods, including the use of bone screws that pass throughcorresponding openings in each of the elements and thread into vertebralbone, and/or by the inclusion of spikes or teeth that penetrate thevertebral endplates to inhibit migration or expulsion of the device. Thearticular components are typically configured to allow the elements, andcorrespondingly the adjacent vertebrae, to pivot and/or rotate relativeto one another.

As discussed above, prior intervertebral disc arthroplasty devices arerelatively difficult to implant between adjacent vertebrae. To implantsuch devices, the adjacent vertebrae are spread apart a distance that issomewhat greater than the normal distance separating the vertebrae sothat the device can be maneuvered between the vertebrae and the anchorscan be engaged to the vertebral endplates. Such an operation presents arisk of injury to the vertebrae caused by misplacement and/or scratchingof the vertebral endplates or other tissue by the anchors. Suchoperation also presents a risk of injury resulting from over-distractionof the intervertebral space. As also discussed above, other types ofprior arthroplasty devices require the threading of bone screws oranother type of fastener into the adjacent vertebrae. However, this typeof anchoring method requires precise placement and orientation of thebone screws to provide adequate anchoring and to avoid injury toadjacent tissue or vertebral structures. Moreover, prior methods ofimplanting arthroplasty devices do not reliably position the device atthe proper location within the intervertebral disc space.

The articular components associated with prior arthroplasty devices arealso prone to wear, particularly in cases where the abutting surfacearea of the articular joint is relatively small. Generally, as theabutting surface area of an articular joint is reduced, contact stressis correspondingly increased which may reduce the overall life of thejoint. As a result, worn out components must be periodically replaced toavoid malfunctioning or potential breakage of the arthroplasty device.

Thus, there is a general need in the industry to provide an improvedarticular disc prosthesis and a method of implanting the same than iscurrently available within the industry. The present invention meetsthis need and provides other benefits and advantages in a novel andunobvious manner.

SUMMARY OF THE INVENTION

The present invention relates generally to an articular disc prosthesisand a method of implanting the same. While the actual nature of theinvention covered herein can only be determined with reference to theclaims appended hereto, certain forms of the invention that arecharacteristic of the preferred embodiments disclosed herein aredescribed briefly as follows.

One form of the present invention is directed to an articular discprosthesis, including a first articular component having a first bearingsurface adapted to engage a first vertebra, and a second articularcomponent having second bearing surface adapted to engage a secondvertebra, with the first and second bearing surfaces defining a spacetherebetween. At least one of the first and second articular componentsincludes a concave articular surface that cooperates with acorresponding convex articular surface to provide articulating motionbetween the first and second articular components, with at least aportion of the concave articular surface extending beyond the spacebetween the first and second bearing surfaces.

Another form of the present invention is directed to an articular discprosthesis for replacement of a natural intervertebral disc, including afirst articular component defining a first concave articular surface, asecond articular component defining a second concave articular surface,and an articular ball positioned between the first and second concavearticular surfaces to provide articulating motion between the first andsecond articular components, and wherein the articular ball has adiameter greater than a height of the natural intervertebral disc.

Another form of the present invention is directed to an articular discprosthesis, including a first articular component adapted to engage afirst vertebra, a second articular component adapted to engage a secondvertebra, and wherein each of the first and second articular componentsextends along an axis and includes a central axial portion defining aconvex lateral curvature and a pair of flanges extending laterally fromthe central axial portion in generally opposite directions.

Another form of the present invention is directed to an articular discprosthesis, including first and second modular articular components,with each of the modular articular components having an outer shellportion configured to engage a corresponding one of first and secondvertebrae, and an inner insert portion removably engaged with the outershell portion. The inner insert portion includes an articular surfacecooperating with a corresponding articular surface to providearticulating motion between the first and second modular articularcomponents.

Another form of the present invention is directed to a method ofimplanting an articular disc prosthesis between first and secondvertebrae, including providing an articular disc prosthesis having afirst articular component adapted to engage a first vertebra and asecond articular component adapted to engage a second vertebra, witheach of the first and second articular components extending along anaxis and including a central axial portion defining a convex lateralcurvature and a pair of flanges extending laterally from the centralaxial portion in generally opposite directions. The method furtherincludes removing at least a portion of a natural interverterbral discfrom between the first and second vertebrae to form an intervertebralspace, forming an elongate recess along a central region of each of thefirst and second vertebrae, and implanting the articular disc prosthesiswithin the intervertebral space by inserting the central axial portionsof the first and second articular components within the elongaterecesses formed along the first and second vertebrae.

It is one object of the present invention to provide an improvedarticular disc prosthesis. It is another object of the present inventionto provide an improved method of implanting an articular disc prosthesiswithin the intervertebral disc space between adjacent vertebral bodies.

Further objects, features, advantages, benefits, and aspects of thepresent invention will become apparent from the drawings and descriptioncontained herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an articular disc prosthesis accordingto one form of the present invention.

FIG. 2 is a front elevational view of the articular disc prosthesisillustrated in FIG. 1.

FIG. 3 is a side elevational view of the articular disc prosthesisillustrated in FIG. 1.

FIG. 4 is an exploded perspective view of the articular disc prosthesisillustrated in FIG. 1.

FIG. 5 is a top view of an endplate according to one embodiment of thepresent invention for use with the articular disc prosthesis illustratedin FIG. 1.

FIG. 6 is an end view of the endplate illustrated in FIG. 5.

FIG. 7 is a side view of the endplate illustrated in FIG. 5.

FIG. 8 is a sectional view of the endplate illustrated in FIG. 5, takenalong line 8—8 of FIG. 5.

FIG. 9 is an end view of an insert according to one embodiment of thepresent invention for use with the articular disc prosthesis illustratedin FIG. 1.

FIG. 10 is a top view of the insert illustrated in FIG. 9.

FIG. 11 is a side view of the insert illustrated in FIG. 10.

FIG. 12 is a sectional view of the insert illustrated in FIG. 9, takenalong line 12—12 of FIG. 9.

FIG. 13 is a lateral view of a portion of the spinal column,illustrating a pair of adjacent upper and lower vertebrae separated by anatural intervertebral disc.

FIG. 14 is an anterior view of the portion of the spinal column shown inFIG. 13, illustrating the removal of portions of the upper and lowervertebrae to accommodate insertion of the articular disc prosthesisillustrated in FIG. 1 therebetween.

FIG. 15 is a lateral view of the portion of the spinal column shown inFIG. 14.

FIG. 16 is an anterior view of the portion of the spinal column shown inFIG. 14, illustrating implantation of the articular disc prosthesisbetween the upper and lower vertebrae.

FIG. 17 is a partial sectional view of the portion of the spinal columnshown in FIG. 16, illustrating implantation of the articular discprosthesis between the upper and lower vertebrae.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiments illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of theinvention is hereby intended, and that any alterations and furthermodifications in the illustrated devices, and any further applicationsof the principles of the invention as illustrated herein arecontemplated as would normally occur to one skilled in the art to whichthe invention relates.

Referring to FIGS. 1–4, shown therein is an articular disc prosthesis 20according to one form of the present invention. The disc prosthesis 20extends generally along a longitudinal axis L and includes an upperarticular component 22 a, a lower articular component 22 b, and anarticular ball 24 disposed between the upper and lower articularcomponents 22 a, 22 b. The articular ball 24 defines a convex articularsurface 25 and preferably has a spherical configuration. However, itshould be understood that the articular ball 24 may take on otherconfigurations as well, such as, for example, an elliptical or eccentricconfiguration.

The articular components 22 a, 22 b are configured substantiallyidentical to one another. Therefore, the description and/or illustrationof one of the articular components 22 a, 22 b applies equally to theother. In a preferred embodiment of the invention, the articularcomponents 22 a, 22 b have a modular configuration. More specifically,each of the articular components 22 a, 22 b preferably includes an outershell or endplate 26 and an inner articular cup or insert 28. As will bediscussed in further detail below, the articular insert 28 can beremoved from the endplate 26 and replaced with an insert of the sametype or of a different type. The articular insert 28 is secured inposition relative to the endplate 26 by a first set of fasteners 30. Asecond set of fasteners 32 is preferably included to engage the firstset of fasteners to prevent the first set of fasteners 30 from looseningand backing out. In one embodiment of the invention, the fasteners 30,32 are threaded set screws. However, it should be understood that othertypes and configurations of fasteners are also contemplated as wouldoccur to one of ordinary skill in the art.

The upper and lower articular components 22 a, 22 b and the articularball 24 cooperate to form an articulating joint that is sized andconfigured for disposition within an intervertebral disc space betweenadjacent vertebral bodies. Specifically, the convex outer surface 25 ofthe articular ball 24 cooperates with corresponding concave surfacesformed in the articular inserts 28 to provide-relative articulatingmotion between the articular components 22 a, 22 b. In a preferredembodiment of the invention, such articulating motion includes bothpivotal and rotational movement to maintain or restore motionsubstantially similar to normal bio-mechanical motion provided by anatural intervertebral disc.

In one embodiment of the invention, the articular components 22 a, 22 bare permitted to rotate relative to one another about a rotational axisR. In another embodiment of the invention, the articular components 22a, 22 b are permitted to pivot relative to one another about a number ofaxes, including lateral or side-to-side pivotal movement about thelongitudinal axis L and anterior-posterior pivotal movement about atransverse axis T. In a further embodiment of the invention, thearticular components 22 a, 22 b are permitted to pivot relative to oneanother about any axis which lies in a plane that intersects thelongitudinal axis L and the transverse axis T. Although the discprosthesis 20 has been illustrated and described as providing acombination of various articulating movements, it should be understoodthat other variations and combinations of articulating movements arealso contemplated as falling within the scope of the present invention.It should also be understood that other types of articulating movementare also contemplated, such as, for example, relative translational orlinear movement.

Although the various components of the articular disc prosthesis 20 maybe formed from a wide variety of materials, following is a listing ofvarious component materials according to one embodiment of the presentinvention. It should be understood, however, that the components of thedisc prosthesis 20 may be formed of materials other than thosespecifically listed below, including any bio-compatible material thatwould be known to one of ordinary skill in the art or any otherequivalent material.

The outer endplates 26 are preferably formed of a polymeric material,such as, for example, a polyaryletherketone polymer or polyethylene. Inother embodiments of the invention, the outer endplates 26 may be formedof titanium, stainless steel, other types of metallic materials, or aceramic material. The outer surfaces of the endplate 26 that areintended to be in direct contact with vertebral bone are preferablycoated with a bone-growth promoting substance, such as, for example, ahydroxyapatite (HA) coating formed of calcium phosphate. The articularinserts 28 are preferably formed of a metallic material, such as, forexample, cobalt-chrome-molybdenum metallic alloy (ASTM F-799 or F-75).In other embodiments of the invention, the articular inserts 28 may beformed of other types of metallic materials, such as, for example,titanium or stainless steel, a ceramic material, or a polymericmaterial. The articular ball 24 is preferably formed of a polymericmaterial, such as, form example, an ultra-high molecular weightpolyethylene (UHMWPE). In another embodiment of the invention, thearticular ball 24 may be cross-linked by radiation, by chemical means orby any other method know to those of skill in the art. In otherembodiments of the invention, the articular ball 24 may be formed oftitanium, stainless steel, other types of metallic materials, or aceramic material. In one embodiment of the invention, the first set offasteners 30 is formed of a polymeric material, such as, for example, apolyaryletherketone polymer. Preferably, the first set of fasteners 30is formed of the same material as the endplates 26. In anotherembodiment of the invention, the second set of fasteners 32 is formed ofa metallic material, such as, for example, cobalt-chrome-molybdenummetallic alloy. Preferably, the second set of fasteners 32 is formed ofthe same material as the articular inserts 28. In other embodiments ofthe invention, the first and second sets of fasteners 30, 32 may beformed of other types of materials, such as, for example, titanium,stainless steel, other types of metallic materials, a ceramic material,or a polymeric material.

Referring to FIGS. 5–8, shown therein are various details regarding theconfiguration of the outer endplates 26. In one embodiment of theinvention, the endplates 26 are each comprised of a central axialportion 50 and a lip portion 52 extending about a periphery of the axialportion 50. The peripheral lip portion 52 is comprised of a pair offlanges or wings 54, 56 extending laterally from the central portion 50,and a flange or lip 58 extending axially from the central portion 50. Asshown in FIG. 5, the outer peripheral profile of the endplate 50 ispreferably sized and shaped to substantially correspond to the size andshape of the vertebral endplate of an adjacent vertebra.

In one embodiment of the invention, the central axial portion 50 has ahemi-cylindrical configuration, including an outer surface 60 defining aconvex lateral curvature extending along an axial length l. It should beunderstood, however, that the central axial portion 50 may take on otherconfigurations, including other types of arcuate configurations, arectangular configuration, or various types of polygonal configurations.It should also be understood that the convex outer surface 60 may takeon other shapes, including an hemi-elliptical shape or other types ofarcuate and/or polygonal configurations. The axial portion 50 includesan open axial end 62, a closed axial end 64, and a concave inner surface66. The concave inner surface 66 defines a cavity 68 extending axiallybetween the open and closed ends 62, 64. As will be discussed below, thecavity 68 is sized and shaped to receive a corresponding portion of thearticular insert 28 therein. In one embodiment of the invention, thecavity 68 has a hemi-cylindrical configuration. However, it should beunderstood that the cavity 68 may take on other configurations as well,including other types of arcuate configurations, a rectangularconfiguration, or other types of polygonal configurations.

The lateral flanges 54, 56 each include an outwardly facing bearingsurface 70 and an inwardly facing surface 72. In one embodiment of theinvention, the outwardly facing bearing surface 70 is contiguous withthe hemi-cylindrical outer surface 60 of the central axial portion 50.Similarly, the inwardly facing surface 72 is contiguous with thehemi-cylindrical inner surface 66 of the central axial portion 50. Itshould be understood, however, that other positions and orientations ofthe lateral flanges 54, 56 relative to the central axial portion 50 arealso contemplated as falling within the scope of the present invention.

The outwardly facing bearing surface 70 preferably defines a number ofanchor elements configured to engage vertebral bone. In one embodimentof the invention, the outwardly facing bearing surface 70 defines anumber of projections or teeth 74. The teeth 74 are preferablytriangular-shaped, defining pointed tips configured to bite into andsecurely grip vertebral bone. However, it should be understood thatother configurations of the teeth 74 are also contemplated as wouldoccur to one of skill in the art. It should also be understood thatother types and configurations of anchor elements are also contemplated,such as, for example, spikes, protrusions, or various types of surfaceroughening features to aid in gripping vertebral bone to inhibitmigration or expulsion of the disc prosthesis 20. In the illustratedembodiment of the invention, the teeth 74 extend laterally across asubstantial portion of the width of flanges 54, 56 and are positionedintermittently along the length of the flange 54, 56. However, inanother embodiment of the invention, the teeth 74 may extend along thelength of the flange 54, 56 and may be positioned intermittently alongthe width of the flange 54, 56. It should also be understood that otherpositions and orientations of the teeth 74 are also contemplated asfalling within the scope of the present invention.

In one embodiment of the invention, each of the lateral flanges 54, 56has a first end 80, a second end 82, and an axial passage 84 extendingfrom the first end 80 toward the second end 82. The axial passage 84 isdisposed in communication with the hollow cavity 68 defined by thecentral axial portion 50, the purpose of which will be discussed below.In one embodiment, the axial passage 84 includes a circular portion 86and a slot portion 88, with the slot portion 88 extending between thehollow cavity 68 and the circular portion 86. Internal threads 87 arepreferably defined along a length of the circular portion 86 of axialpassage 84 which are configured to threadingly receive the first andsecond sets of fasteners 30, 32.

The axial lip 58 includes an outwardly facing surface 90 and an inwardlyfacing surface 92. In one embodiment of the invention, the outwardlyfacing surface 92 is substantially flat. However, it should beunderstood the outwardly facing surface 92 could alternatively define anumber of anchor elements configured to engage vertebral bone. The axiallip 58 includes an axially facing end surface 94 extending between thelateral flanges 54, 56. In one embodiment, the axially facing endsurface 94 defines a recessed area 96 extending inwardly toward thecentral portion 50, the purpose of which will become apparent below. Therecessed area 96 preferably has an arcuate configuration; however, otherconfigurations are also contemplated as would occur to one of skill inthe art.

As illustrated in FIGS. 6 and 8, in one embodiment of the invention, theinwardly facing surfaces 72 of the lateral flanges 54, 56 preferablydefine an outward taper arranged at a taper angle α. The outward taperpreferably extends in a lateral direction along the entire length of theflanges 54, 56 (as shown in FIG. 6). Additionally, at least the endportions of the inwardly facing surface 72 adjacent the ends 80, 82 arepreferably tapered in an axial direction at a taper angle α. (as shownin FIG. 8). The inwardly facing surface 92 of the axial lip 58 alsopreferably defines an outward taper extending in an axial direction at ataper angle α. As should be appreciated, the inwardly facing surfaces72, 92 of the lateral flanges 54, 56 and the axial lip 58 cooperate todefine a substantially conically-shaped surface surrounding the centralaxial portion 50 and extending outwardly relative to the rotational axisR at the taper angle α. In this manner, relative pivotal movementbetween the articular components 22 a, 22 b is limited to apredetermined range of motion via abutment of the inwardly facingsurfaces 72, 92 of one of the endplate 26 against the inwardly facingsurfaces 72, 92 of the opposing endplate 26.

In one embodiment of the invention, the taper angle α falls within arange of between about 5 degrees and about 15 degrees, thereby limitingrelative pivotal motion between the articular components 22 a, 22 bwithin a range of just over 10 degrees to just over 30 degrees. In amore specific embodiment, the taper angle a is about 7.5 degrees,thereby limiting relative pivotal motion between the articularcomponents 22 a, 22 b to just over 15 degrees. It should be understood,however, that the taper angle α may take on other values to satisfy thespecific articular requirements of the disc prosthesis 20, includingtaper angles α less than 5 degrees and greater than 15 degrees. Itshould also be understood that the taper angle α need not necessarily beuniform, but may instead be varied to limit relative pivotal motionbetween the articular components 22 a, 22 b within different rangesdepending upon the particular pivotal axis about which the articularcomponents 22 a, 22 b are being pivoted. In the illustrated embodimentof the invention, each of the endplates 26 of the articular components22 a, 22 b define outwardly tapering surfaces 72, 92. However, it shouldbe understood that in another embodiment of the invention, only one ofthe endplates 26 defines outwardly tapering surfaces 72, 92, with theother endplate 26 defining substantially flat inwardly facing surfaces72, 92. In a further embodiment of the invention, both of the endplates26 define substantially flat inwardly facing surfaces 72, 92.

Referring to FIGS. 9–12, shown therein are various details regarding theconfiguration of the inner articular inserts 28. In one embodiment ofthe invention, the articular inserts 28 are each comprised of a centralbody 100 and a pair of splines 102, 104 extending laterally from thecentral body 100. The central body 100 preferably has a shape andconfiguration that corresponds to the shape and configuration of theinner cavity 68 of the endplate 26. In one embodiment, the central body100 includes a convex outer surface 106 that corresponds to the concaveinner surface 66 of the endplate 26. The central body 100 also includesa relatively flat inner surface 108 disposed generally opposite theconvex outer surface 106, and a pair of opposite axial end surfaces 110,112. An axial opening 124 is preferably formed through the end surface112 which is configured to receive a portion of an insertion instrumentor tool therein (not shown). As illustrated in FIG. 10, the central body100 has a hemi-cylindrical configuration that closely corresponds to thehemi-cylindrical configuration of the inner cavity 68 of the endplate26. It should be understood, however, that the central body 100 may takeon other configurations, including other types of arcuateconfigurations, a rectangular configuration, or various types ofpolygonal configurations.

The central body 100 includes a relatively large recess or socket 120extending from the flat inner surface 108. The socket 120 defines aconcave articular surface 122 that cooperates with the convex articularsurface 25 of the ball 24 to provide articulating motion between thearticular components 22 a, 22 b. More particularly, the ball 24 is atleast partially disposed within the socket 120 such that the convex andconcave articular surfaces 25, 122 are positioned in abutment to allowpivotal and rotational movement therebetween. In a preferred embodimentof the invention, the socket 120 is shaped and configured to closelycorrespond to the shape and configuration of the articular ball 24. In aone embodiment, the convex surface 25 of the ball 24 has a radius thatis substantially equal to the radius of curvature of the concave surface122 of socket 120. However, it should be understood that the radius ofthe articular ball 24 may be sized somewhat smaller than the radius ofcurvature of the socket 120. In one embodiment of the invention, thediameter of the articular ball 24 falls within a range of about 10 mm toabout 30 mm. In a more specific embodiment, the diameter of thearticular ball 24 is approximately 19 mm. Notably, since the area ofabutment between the convex surface 25 of the articular ball 24 and theconcave surface 122 of the socket 120 is relatively large, internalstresses within the disc prosthesis 20 are spread out over an increasedsurface area, thereby resulting in decreased wear and prolonged designlife of the articular ball 24 and/or the articular inserts 28. Moreover,reducing internal stresses within the disc prosthesis 20 provides anopportunity to form the articular ball 24 and/or the articular inserts28 from nonmetallic materials, such as, for example, a polymericmaterial or a ceramic material.

Although the articular ball 24 and the socket 120 are illustrated ashaving generally smooth, uninterrupted abutting articular surfaces 25,122, it should be understood that in other embodiments of the invention,either or both of the articular surfaces 25, 122 may define one or moresurface depressions to facilitate removal of matter disposed betweenabutting portions of the articular surfaces. Such surface depressionsmay include, for example, grooves, channels, passages, openings,flattened areas, or dimples. Further details regarding the inclusion ofsurface depressions on either or both of the articular surfaces 25, 122are disclosed in co-pending U.S. patent application Ser. No. 10/042,589,filed on Jan. 9, 2002 and entitled “Intervertebral Prosthetic Joint”,the contents of which are hereby expressly incorporated by reference intheir entirety.

The splines 102, 104 extending from the central body 100 are shaped andconfigured to be received within the axial passages 84 extending throughthe flanges 54, 56 of the endplate 26. Each of the splines 102, 104preferably includes a first axial portion 130 and a second axial portion132. The first axial portion 130 has a lateral width that is somewhatgreater than the lateral width of the second axial portion 132 so as toform an axially facing shoulder 134, the purpose of which will bediscussed below. As shown in FIGS. 6 and 9, the overall axial profile ofthe articular insert 28 substantially corresponds to that of the cavity68 and the axial passages 84 defined within the endplate 26.

Referring once again to FIG. 4, the articular components 22 a, 22 b ofthe disc prosthesis 20 are assembled by engaging the articular inserts28 with the endplates 26. More specifically, the articular insert 28 isaxially inserted into the endplate 26, with the central body 100 and thesplines 102, 104 of the insert 28 being slidably displaced along thecentral cavity 68 and the axial passages 84 of the endplate 26. Thearticular insert 28 is retained within the endplate 26 by way of thefirst set of set screws 30. The set screws 30 are threadingly engagedalong the threaded portion 87 of the axial passage 84 until tightlyengaged against the axial shoulder 134 of the splines 102, 104. Thesecond set of set screws 32 are then threadingly engaged along thethreaded portion 87 of the axial passage 84 until the set screws 32engage the first set of set screws 30. The second set of set screws 32serve to prevent the first set of set screws 30 from loosening andbacking out. Once the articular components 22 a, 22 b have beenassembled, the articular ball 24 is positioned within the sockets 120defined by the articular inserts 28 to form the articulating discprosthesis 20.

It should be appreciated that the modular nature of the disc prosthesis20 offers several advantages. For example, if either of the articularcomponents 22 a, 22 b or the articular ball 24 begins to malfunction orexhibits signs of wear, the disc prosthesis 20 can be easilydisassembled by simply removing the set screws 30, 32 and sliding thearticular inserts 28 and the articular ball 24 out from the endplates26. Notably, removal of the articular inserts 28 and the articular ball24 can be done in situ without having to remove the endplates 26 fromthe intervertebral disc space. This is particularly advantageous if boneon-growth onto the endplates 26 has already commenced, thereby avoidinghaving to break the bony engagement between the endplates 26 andadjacent vertebral bone.

The modular nature of the disc prosthesis 20 also allows thearticulating characteristics and movements to be revised without havingto remove the entire disc prosthesis 20 from the intervertebral discspace. Notably, the articular components 22 a, 22 b and the articularball 24 originally implanted within the intervertebral disc space can beremoved from the endplates 26 and replaced with differenttypes/configurations of articular inserts 28 and/or a differentarticular ball 24 designed to provide the disc prosthesis 20 withmodified articulating characteristics and movements. Once again, removalof the articular inserts 28 and the articular ball 24 can be done insitu without removing the endplates 26 from the intervertebral discspace. Additionally, the articular components 22 a, 22 b and thearticular ball 24 may be removed from the endplates 26 and replaced witha rigid spacer element to provide rigid stabilization between theadjacent vertebrae, or by a semi-rigid or flexible spacer element toprovide flexible stabilization between the adjacent vertebrae.

Referring to FIG. 13, shown therein is a lateral view of a portion ofthe spinal column, illustrating a pair of adjacent upper and lowervertebrae V_(U), V_(L) separated by a natural intervertebral disc D. Asdiscussed above, in cases where the natural intervertebral disc D isdiseased or degenerated, most if not all of the natural disc D istypically removed via a discectomy or a similar surgical procedure, thedetails of which would be known to one of ordinary skill in the art.

As illustrated in FIGS. 14 and 15, removal of the diseased ordegenerated disc D results in the formation of an intervertebral discspace S between the upper and lower vertebrae V_(U), V_(L). Toaccommodate for the insertion of the disc prosthesis 20 within theintervertebral disc space S, preparation of the upper and lowervertebrae V_(U), V_(L) is required. In one embodiment of the invention,the intervertebral space S is enlarged by forming elongate openings orrecesses 300 along the inferior and superior portions of the upper andlower vertebrae V_(U), V_(L), respectively. The elongate recesses 300preferably have a shape and configuration that substantially correspondsto the outer profile of the central axial portions 50 of the articularcomponents 22 a, 22 b. In one embodiment, the elongate recesses 300 havea hemi-cylindrical shape; however, other shapes and configurations ofthe recesses 300 are also contemplated as would occur to one of skill inthe art, including other types of arcuate configurations, a rectangularconfiguration, or other types of polygonal configurations.

In one embodiment of the invention, the elongate recesses 300 extendfrom an anterior side 302 of the vertebrae V_(U), V_(L) toward aposterior side 304 of the vertebrae V_(U), V_(L) to a predetermineddepth d. In a preferred embodiment of the invention, the predetermineddepth d of the elongate recesses 300 is approximately equal to orslightly greater than the length l of the central axial portions 50 ofthe articular components 22 a, 22 b. As will be discussed in furtherdetail below, forming the recesses 300 at a predetermined depth dcorrespondingly controls the insertion depth of the disc prosthesis 20to ensure proper positioning of the disc prosthesis 20 within theintervertebral disc space S. In one embodiment of the invention, theelongate recesses 300 are formed by reaming. However, other methods offorming the recesses 300 are also contemplated as would occur to one ofordinary skill in the art, such as, for example, by drilling, chiselingor curetting.

Referring to FIGS. 16 and 17, following preparation of the upper andlower vertebrae V_(U), V_(L), the disc prosthesis 20 may then beimplanted within the intervertebral disc space S. In one embodiment ofthe invention, implantation is accomplished by inserting the cylindricalaxial portions 50 of the articular components 22 a, 22 b within theelongate recesses 300, with the bearing surfaces of the lateral flanges54, 56 and the axial lip 58 facing the vertebral endplates of the upperand lower vertebrae V_(U), V_(L). The end surface 94 of the axial lip 58faces a posterior direction, with the recessed area 96 defined by theaxial lip 58 (FIG. 5) providing sufficient clearance to avoidencroachment into the area adjacent the spinal canal.

Prior to implantation of the disc prosthesis 20 within theintervertebral disc space S, the articular components 22 a, 22 b arepreferably placed in a predetermined relationship with respect to oneanother. In one embodiment of the invention, an insertion instrument(not shown) may be used to position and secure the articular components22 a, 22 b at a predetermined spacing and at a predetermined orientationrelative to one another. The insertion instrument would maintain thearticular components 22 a, 22 b at the predetermined spacing andorientation during manipulation of the disc prosthesis 20, and would becapable of selectively releasing the disc prosthesis 20 once properlypositioned within the intervertebral disc space S. Such insertioninstrument may include, for example, a pair of prongs adapted forinsertion within the axial openings 124 formed in the articular inserts28 of the articular components 22 a, 22 b.

As should be appreciated, the specific angular relationship between thearticular component 22 a, 22 b is dictated by the geometry of the upperand lower vertebrae V_(U), V_(L) and the particular curvature orlordosis of the portion of the spinal column being treated. As such, therelative angular orientation of the planes P₁ and P₂ defined along thebearing surfaces 70 of the endplate flanges 54, 56 should correspond tothe particular geometric configuration of the natural intervertebraldisc D. As should also be appreciated, the distance between the planesP₁ and P₂ should be approximately equal to the height of the naturalintervertebral disc D. Additionally, although the bearing surfaces 70 ofthe endplate flanges 54, 56 have been illustrated and described ashaving a substantially planar configuration, it should be understoodthat the bearing surfaces 70 may take on other configurations. Forexample, the bearing surfaces 70 may take on a curved or arcuateconfiguration that corresponds to the particular contour of the adjacentvertebral endplate against which the bearing surfaces 70 are engaged.

In one embodiment of the invention, the disc prosthesis 20 is insertedbetween the upper and lower vertebrae V_(U), V_(L) in a directiongenerally parallel to its longitudinal axis L, with the central axialportions 50 of the endplates 26 being axially displaced through theelongate recesses 300. Notably, since the central axial portions 50 areaxially displaced through the preformed recesses 300, distraction of theupper and lower vertebrae V_(U), V_(L) to accommodate insertion of thedisc prosthesis 20 is minimized, if not eliminated entirely. In theillustrated embodiment of the invention, the disc prosthesis 20 isinserted into the intervertebral disc space S via an anterior approach.However, it should be understood that the elongate recesses 300 mayalternatively extend from the posterior side 304 of the vertebrae V_(U),V_(L) toward the anterior side 302 at a predetermined depth d toaccommodate insertion of the disc prosthesis 20 into the intervertebraldisc space S via a posterior approach. It should also understood thatthe elongate recesses 300 may alternatively extend from a first lateralside of the vertebrae V_(U), V_(L) toward an opposite lateral side ofthe vertebrae at a predetermined depth d to accommodate insertion of thedisc prosthesis 20 into the intervertebral disc space S via a lateralapproach.

As discussed above, the depth d of the elongate recesses 300 isapproximately equal to or slightly greater than the length l of thecentral axial portions 50 of the endplates 26. Accordingly, preciseposition of the disc prosthesis 20 within the intervertebral disc spaceS is possible. Specifically, proper axial positioning of the discprosthesis 20 is accomplished when the insertion ends 64 of the centralaxial portions 50 bottom out against the axially facing end surfaces 301of the elongate recesses 300. Controlling the insertion depth of thedisc prosthesis 20 results in more precise positioning to avoidover-insertion or under-insertion of the disc prosthesis 20.Additionally, disposition of the central axial portions 50 within theelongate recesses 300 substantially prevents lateral and/or rotationalmovement of the articular components 22 a, 22 b with respect to theupper and lower vertebrae V_(U), V_(L). The relatively large surfacearea of the central axial portions 50 contacting the upper and lowervertebrae V_(U), V_(L) also tends to minimize subsidence into thecancellous bone. Moreover, engagement of the bearing surfaces of thelateral flanges 54, 56 and the axial lip 58 against the upper and lowervertebrae V_(U), V_(L) tends to minimize subsidence of the discprosthesis 20 into the cancellous bone.

Once the articular components 22 a, 22 b are properly positioned withinthe intervertebral disc space S, the axial lip 58 of the endplates 26will bear against the posterior cortical rim of the upper and lowervertebrae V_(U), V_(L). Additionally, the anterior ends of the centralaxial portions 50 will bear against the anterior cortical rim of theupper and lower vertebrae V_(U), V_(L). Moreover, the lateral flanges54, 56 may be configured to bear against the lateral cortical rim and/orthe anterior cortical rim of the upper and lower vertebrae V_(U), V_(L).Such bearing engagement between the endplates 26 of the articularcomponents 22 a, 22 b and the outer rim of the upper and lower vertebraeV_(U), V_(L) provides additional stabilization of the disc prosthesis 20and tends to minimize subsidence. Additionally, the teeth 74 formedalong the lateral flanges 54, 56 grip the bony endplates of the upperand lower vertebrae V_(U), V_(L) to resist migration of the discprosthesis 20 and/or to prevent expulsion of the disc prosthesis 20 fromthe intervertebral disc space S.

The disc prosthesis 20 is initially maintained in position within theintervertebral disc space S relative to the upper and lower vertebraeV_(U), V_(L) via disposition of the central axial portions 50 within theelongate recesses 300 and by engagement of the teeth 74 against the bonyvertebral endplates. However, over time the disc prosthesis 20 will befurther secured to the upper and lower vertebrae V_(U), V_(L) via bonyon-growth onto the surfaces of the articular components 22 a, 22 b thatare in contact with vertebral bone tissue, particularly those surfaceswhich are in contact with metabolically active cancellous bone. Suchbony on-growth provides further resistance to migration of the discprosthesis 20 and possible expulsion from the intervertebral disc spaceS. It should be understood that other means for engaging the discprosthesis 20 to the upper and lower vertebrae V_(U), V_(L) are alsocontemplated, such as, for example, bone screws, staples, an adhesive,or by other methods of engagement that would occur to one of ordinaryskill in the art.

In use, the articular components 22 a, 22 b and the articular ball 24cooperate to provide a ball-and-socket type joint that permits relativepivotal and rotational movement between the articular components 22 a,22 b, which correspondingly permits relative pivotal and rotationalmovement between the upper and lower vertebrae V_(U), V_(L). Morespecifically, the spherical surface 25 of the articular ball 24 isslidably engaged against the concave surfaces 122 of the articularinserts 28. The resulting pivotal and rotational movement of thearticular components 22 a, 22 b serves to maintain or restore articularmotion to the portion of the spinal column being treated that issubstantially similar to the normal bio-mechanical motion provided by anatural intervertebral disc D.

As shown in FIGS. 16 and 17, the unique geometry of the articularcomponents 22 a, 22 b allows the use of a relatively large articularball 24. As discussed above, use of an articular ball 24 having a largediameter increases the area of abutment between the convex surface 25 ofthe ball 24 and the concave surface 122 of the socket 120. As a result,internal stresses within the disc prosthesis 20 are reduced, therebyresulting in decreased wear and prolonged design life of the discprosthesis 20. Use of a relatively large diameter articular ball 24 ismade possible by the hemi-cylindrical central portions 50 of theendplates 26 which are positioned within the hemi-cylindrical recesses300 formed along the upper and lower vertebrae V_(U), V_(L). Notably,this unique geometric design allows the use of an articular ball 24having a diameter greater than the height of the natural intervertebraldisc D. As a result, at least a portion of the abutting articularsurfaces 25, 122 of the ball 24 and the articular insert 28 ispositioned beyond the intervertebral disc space defined between theplanes P₁ and P₂ extending along the bearing surfaces 70 of the endplateflanges 54, 56.

Although the disc prosthesis 20 has been illustrated and described asincluding a pair of articular components 22 a, 22 b having a separatearticular ball 24 disposed therebetween, in an alternative embodiment ofthe invention the articular ball 24 may be replaced by a protrusionformed integral with one of the articular inserts 28. In thisalternative embodiment, the protrusion extending from one of thearticular inserts 28 would be at least partially disposed within thesocket 120 defined by the opposing articular insert 28. A convexarticular surface defined by the protrusion would cooperate with theconcave articular surfaces 122 defined by the opposing socket 120 toprovide pivotal and rotational articulating motion between the articularcomponents 22 a, 22 b.

Additionally, although the devices and methods illustrated and describedabove are particularly useful in treating the lumbar region of thespine, it should nevertheless be understood that the present inventionis also applicable to other portions of the spine, including thecervical or thoracic regions of the spine.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, the same is to be considered asillustrative and not restrictive in character, it being understood thatonly the preferred embodiments have been shown and described and thatall changes and modifications that come within the spirit of theinvention are desired to be protected.

1. An articular disc prosthesis for disposition between a first vertebraand a second vertebra, comprising: a first articular component includinga first cylindrical portion and a first flange portion extendinglaterally therefrom and defining a first bearing surface adapted toengage the first vertebra; and a second articular component including asecond cylindrical portion and a second flange portion extendinglaterally therefrom and defining a second bearing surface adapted toengage the second vertebra, said first and second bearing surfacesdefining a space therebetween; and wherein at least one of said firstand second articular components includes a concave articular surfacethat cooperates with a corresponding convex articular surface to providearticulating motion between the first and second articular components,at least a portion of said concave articular surface positioned beyondsaid space.
 2. The articular disc prosthesis of claim 1, wherein each ofsaid first and second articular components includes a concave articularsurface that cooperates with said corresponding convex articular surfaceto provide said articulating motion, at least a portion of each of saidconcave articular surface positioned beyond said space.
 3. The articulardisc prosthesis of claim 2, further comprising an articular ballpositioned between said first and second articular components, saidarticular ball cooperating with each of said concave articular surfacesto provide said articulating motion.
 4. The articular disc prosthesis ofclaim 1, wherein said first and second bearing surfaces include meansfor gripping vertebral bone.
 5. An articular disc prosthesis fordisposition between a first vertebra and a second vertebra, comprising:a first articular component adapted to engage the first vertebra; and asecond articular component adapted to engage the second vertebra; andwherein each of said first and second articular components extends alongan axis and includes a central axial portion defining a convex lateralcurvature and a pair of flanges extending laterally from said centralaxial portion in generally opposite directions.
 6. The articular discprosthesis of claim 5, wherein said central axial portion has ahemi-cylindrical shape.
 7. The articular disc prosthesis of claim 5,wherein each of said pair of flanges includes an outwardly facingbearing surfaces configured to engage a respective one of the first andsecond vertebrae.
 8. The articular disc prosthesis of claim 7, whereinsaid outwardly facing bearing surfaces include means for grippingvertebral bone.
 9. The articular disc prosthesis of claim 5, whereinsaid pair of flanges of said first articular component defines a firstpair of inwardly facing surfaces, said pair of flanges of said secondarticular component defining a second pair of inwardly facing surfacesarranged generally opposite said first pair of inwardly facing surfaces,at least one of said first and second pairs of inwardly facing surfacesdefining an outward taper.
 10. The articular disc prosthesis of claim 9,wherein said outward taper extends in a lateral direction.
 11. Thearticular disc prosthesis of claim 10, wherein said outward taperextends in an axial direction.
 12. The articular disc prosthesis ofclaim 9, wherein each of said first and second pairs of inwardly facingsurfaces defines an outward taper.
 13. The articular disc prosthesis ofclaim 5, further comprising a spherical-shaped ball disposed betweensaid first and second articular components to provide articulatingmotion therebetween, said spherical-shaped ball being at least partiallypositioned within said central axial portion of said first and secondarticular components.
 14. The articular disc prosthesis of claim 5,wherein each of said first and second articular components includes alip extending axially from said central axial portion, said lipincluding an outwardly facing bearing surface configured to engage arespective one of the first and second vertebrae.
 15. The articular discprosthesis of claim 14, wherein said lip includes an axially facingsurface, said axially facing surface defining a recessed area extendinginwardly toward said central axial portion.
 16. An articular discprosthesis for disposition between a first vertebra and a secondvertebra, comprising: a first modular articular component; and a secondmodular articular component; and wherein each of said first and secondmodular articular components includes: an outer shell portion configuredto engage a corresponding one of the first and second vertebrae; aninner insert portion removably engaged with the outer shell portion,said inner insert portion including an articular surface cooperatingwith a corresponding articular surface to provide articulating motionbetween said first and second modular articular components; and anarticular ball positioned between said first and second modulararticular components; and wherein said articular surface of each of saidinner insert portions is a concave articular surface, said articularball cooperating with said concave articular surfaces to provide saidarticulating motion.
 17. The articular disc prosthesis of claim 16,wherein said inner insert portions are formed of a first material, saidarticular ball formed of a second material different from said firstmaterial.
 18. The articular disc prosthesis of claim 17, wherein one ofsaid first and second materials is a metallic material, and wherein theother of said first and second materials is a plastic material.
 19. Thearticular disc prosthesis of claim 16, wherein said inner insert portionis slidably disposed within said outer shell portion and is securelyengaged thereto by a number of fasteners.
 20. A method of implanting anarticular disc prosthesis between first and second vertebrae,comprising: providing an articular disc prosthesis having a firstarticular component adapted to engage a first vertebra and a secondarticular component adapted to engage a second vertebra, each of thefirst and second articular components extending along an axis andincluding a central axial portion defining a convex lateral curvatureand a pair of flanges extending laterally from the central axial portionin generally opposite directions; removing at least a portion of anatural interverterbral disc from between the first and second vertebraeto form an intervertebral space; forming an elongate recess along acentral region of each of the first and second vertebrae; and implantingthe articular disc prosthesis within the intervertebral space byinserting the central axial portions of the first and second articularcomponents within the elongate recesses of the first and secondvertebrae.
 21. The method of claim 20, wherein the inserting comprisesaxially displacing the central axial portions of the first and secondarticular components along the elongate recesses in the first and secondvertebrae.
 22. The method of claim 20, further comprising controllingthe forming of the elongate recesses to a predetermined depth.
 23. Themethod of claim 22, wherein the central axial portions of the first andsecond articular components have an axial length substantially equal tothe predetermined depth of the elongate recesses.
 24. The method ofclaim 23, wherein the forming of the elongate recesses comprisesreaming.
 25. The method of claim 20, further comprising engaging thepair of flanges of each of the first and second articular componentsagainst the vertebral endplate of a corresponding one of the first andsecond vertebrae.
 26. The method of claim 20, wherein the central axialportion of the first and second articular components and the elongaterecesses formed in the first and second vertebrae each have ahemi-cylindrical configuration.
 27. An articular disc prosthesis forreplacement of a natural intervertebral disc, comprising: a firstarticular component defining a first concave articular surface; a secondarticular component defining a second concave articular surface; and anarticular ball positioned between said first and second concavearticular surfaces to provide articulating motion between the first andsecond articular components, said articular ball having a height greaterthan a height of the natural intervertebral disc; wherein each of saidfirst and second articular components includes a first portion definingsaid concave articular surface and a second portion extending laterallyfrom the first portion and defining an outwardly facing vertebralbearing surface, said outwardly facing vertebral bearing surfacesdefined by said first and second articular components separated by adistance substantially equal to the height of the natural intervertebraldisc; and wherein said first portion of said first and second articularcomponents extends along an axis and defines a convex lateral curvature,a pair of said second portions extending laterally from said centralaxial portion in generally opposite directions.
 28. The articular discprosthesis of claim 27, wherein said first portion has ahemi-cylindrical shape.
 29. An articular disc prosthesis for replacementof a natural intervertebral disc, comprising: a first articularcomponent defining a first concave articular surface; a second articularcomponent defining a second concave articular surface; and an articularball positioned between said first and second concave articular surfacesto provide articulating motion between the first and second articularcomponents, said articular ball having a height greater than a height ofthe natural intervertebral disc; wherein each of said first and secondarticular components includes a first portion defining said concavearticular surface and a second portion extending laterally from thefirst portion and defining an outwardly facing vertebral bearingsurface, said outwardly facing vertebral bearing surfaces defined bysaid first and second articular components separated by a distancesubstantially equal to the height of the natural intervertebral disc;and wherein said pair of second portions of said first articularcomponent defines a first pair of inwardly facing surfaces, said pair ofsecond portions of said second articular component defining a secondpair of inwardly facing surfaces arranged generally opposite said firstpair of inwardly facing surfaces, at least one of said first and secondpairs of inwardly facing surfaces defining an outward taper.
 30. Thearticular disc prosthesis of claim 3, wherein said articular ball isspherical-shaped and defines an outer diameter.